1. Field of the Invention
The present invention relates to an ejection detection device and an ejection detection method for detecting the nozzle condition, such as an ejection failure, for a liquid ejection head where a plurality of nozzles are arranged to eject a liquid, and a printing apparatus that employs this ejection detection device.
2. Description of the Related Art
A liquid ejection printing apparatus ejects, to a printing medium, liquid droplets, such as ink droplets, from a plurality of nozzles provided for a liquid ejection head, and prints images. Therefore, the ejection conditions of the individual nozzles of the liquid ejection head have a great effect on the quality of an image. For maintaining the quality of printed images, the liquid ejection printing apparatus employs an ejection detection device that detects the condition wherein liquid droplets were ejected from the individual nozzles, and periodically examines the ejection of liquid droplets to detect the conditions of the nozzles, such as ejection failure.
An example ejection detection device includes: a light-emitting device (LED) that serves as a light-emitting unit for emitting detection light that intersects the flight path of liquid droplets; and a light-receiving device (photodiode) that serves as a light-receiving unit for receiving the detection light. This ejection detection device detects a change in the quantity of light received by the light-receiving device when the ejected liquid droplets have passed the optical path, and determines the presence/absence of an ejection-defective nozzle. That is, a timing at which liquid droplets were ejected and a timing at which the change of the light quantity caused by the interception of light by liquid droplets was detected by the light-receiving device can be employed to determine the passage of liquid droplets, i.e., to determine the presence/absence of an ejection-defective nozzle.
Since all the nozzles of the nozzle array of the liquid ejection head should be settled within the range of the path of detection light formed by the light-emitting device and the light-receiving device, the distance between these devices is extended as the number of nozzles is increased. Therefore, from the viewpoint of reducing space required by a detection mechanism, the light-emitting device and the light-receiving device are arranged at the smallest distance in which the nozzle array can be settled.
Meanwhile, since the detection light rays emitted by the light-emitting device are diverging light rays, the rays intercepted by liquid droplets that were ejected by the nozzles near the light-emitting device are easily affected by optical diffraction. Therefore, according to a correlation between the distance from the nozzles to the light-emitting device and the optical diffraction, the change of the received light quantity that occurs depending on whether ink droplets were ejected tends to be reduced when the nozzles are located near the light-emitting device because the rays to be intercepted by liquid droplets reach the light-receiving device because of diffraction. Furthermore, since the radiant intensity of light is reduced inversely proportionally to the square of the distance, the quantity of light tends to decrease and the quantity of light received by the light-receiving device tends to be reduced, regardless of whether liquid droplets were ejected, when the nozzles are located a distance from the light-emitting device. As shown in FIG. 14 that is a schematic diagram showing a relationship between the locations of nozzles of a conventional ejection detection device and a received photocurrent, since the change of the received light quantity that occurs depending on whether liquid droplets were ejected is reduced for the nozzles located nearer the light-emitting device (LED), the value of the detection output that corresponds to the change of the quantity of received light is gradually lowered. The detection output value indicating a change in the quantity of received light as described above is also gradually reduced for the ejection nozzles located a longer distance from the light-emitting device (or closer to the light-receiving device).
As described above, the detection output level of the light-receiving device greatly differs depending on the locations of nozzles, from which a liquid droplet to be detected is ejected, i.e., a location near the light-emitting device, a location that is far from the light emitting device and that is near the light-receiving device, and an intermediate location, and there is a problem that uniform and stable detection cannot be performed for all of the nozzles.
In order to resolve this problem, a technique for optically detecting the ejection conditions of liquid droplets is disclosed in Japanese Patent Laid-Open No. 2006-007447. According to this technique, a large number of droplets are ejected from the nozzles near the light emission side, while a small number of droplets are ejected from the nozzles near the light reception side, so that almost the same detection output level is obtained at the time of detection for ink droplets. Further, the technique for an ejection failure detection unit employing a laser system is disclosed in Japanese Patent Laid-Open No. 2010-253771, whereby the diameter of ink droplets to be ejected is increased based on the distance between nozzles and a sensor.
However, there is a problem for the techniques in Japanese Patent Application laid-open No. 2006-007447 and Japanese Patent Application Laid-open No. 2010-253771 that the structure that controls the number of ejected droplets or the diameter of droplets only for detection must be prepared, and there is another problem that more consumption of liquid that does not contribute into printing is required. Moreover, since the number of droplets to be ejected or the diameter of an ink droplet employed for the detection process differ from those employed for the actual printing process, the number of ejected droplets or the diameter of the droplets are reduced in the actual printing although ejection of ink droplets was normally performed during the process of detecting ejection conditions, and therefore, there is possibility that an ejection failure will occur when energy to be supplied to the printing elements (heaters or piezoelectric elements) of the individual nozzles is reduced.